Fractal analysis of the electromyographic interference pattern

Evaluation of motor unit recruitment is an important component of the clinical EMG exam. Typically this is assessed qualitatively using auditory features and estimates of the visual complexity of the EMG waveform. Recent advances in nonlinear dynamics have led to the development of the concept of fractals which can be used to quantify complexity and space filling features of various structures. This study was undertaken to determine if the normal EMG interference pattern (IP) has fractal characteristics that might be helpful in quantitative analysis. EMG activity was recorded from the 9 normal biceps muscles as force was varied from 10 to 90% of maximal. Using a box count algorithm, the fractal dimension was calculated. The EMG IP displays fractal characteristics with a dimension that is highly correlated with force and ranges from 1.1 to 1.4 as force increases from 10 to 90% MVC. The fractal dimension (FD)-force relationship is similar to that observed with other methods of IP analysis and suggests that the fractal dimension can be used to quantify and capture the essence of the ‘complexity’ of motor unit recruitment patterns.